Model systems for metalloprotein metal sites will be characterized using various physical techniques such as variable-temperature magnetic susceptibility, and variable-temperature electron paramagnetic resonance, infrared, Raman, 57Fe Mossbauer, and electronic absorption spectroscopies. Major emphasis is placed on understanding the fundamental nature of electron transfer (or electron dynamics) between transition metal ions. Various aspects of electron-transfer proteins are being modeled. Mixed-valence metallocenes are being studied to detail the nature of the mixed-valence character of certain ferredoxins. One objective is to understand whether the electron transfer in the mixed-valence site is occurring in a thermally assisted process by a direct interaction between the Fe(II) and Fe(III) ions or by a "superexchange-type" of interaction involving the bridging ligands or if the electron transfer is not thermal, but occurs by electron-tunneling. To better understand how the spin-flipping in various spin-equilibrium ferric heme proteins might be coupled to electron transport, the details of the electronic structure and the spin-flipping rates will be investigated for a series of spin-equilibrum ferric hemes and a series of model complexes. Magnetic exchange interactions in various Cu(II) and Ni(II) dimeric complexes will be investigated with epr and susceptibility measurements. It is shown that such an approach is a means of gauging the viability of an extended molecular unit for electron transfer. It is planned to study magnetic exchange between two metal ions as propagated by a variety of dihydroxy-benzoquinones including some very extended species. Water as bridge for electron transfer will also be studied. Electron transfer (dynamics) between a metal ion bound at the periphery and the central metal ion in a porphyrin will be studied as a model for electron transfer to and from the iron ion in cytochrome-c.